1. Field of Invention
This invention relates to an improved method, apparatus and related process for determining cloud cover conditions and cloud distribution in a region, and more particularly to a method and apparatus for making such a determination in the vicinity of an airport or airfield.
2. Description of Related Art
An ability to determine and report the distribution and amount of cloud coverage in the vicinity of an airport or airfield is of obvious importance to aircraft flight safety.
Clouds form where temperature and water content induce condensation of water vapor into droplets. In some instances, local heating causes air to rise. As it rises, the cooling process that occurs results in condensation and cloud formation. In other instances, wind distributions and the thermal structure of atmospheric weather systems cause boundaries with abrupt temperature changes. At these boundaries, clouds form due to both temperature and pressure effects. Many clouds assume a form referred to as a stratoform at these boundaries. That is, the clouds are naturally layered or stratified. Stratoform clouds and the somewhat less frequent convective cloud structure (tower cumulus, thunder cells, et cetera) are separate components of sky conditions. In determining and reporting sky conditions it is highly desirable to distinguish between these cloud types.
Cloud coverage measurements are frequently made by trained human observers or automatic instrumentation to determine sky conditions as described in Chapter 12--Sky Conditions, in the document entitled Surface Weather Observing--METAR, Department of Transportation, Federal Aviation Administration, document number 7900.5A, Washington, D.C., July 1996.
Human observers view a region around their location and determine, perhaps with the aid of the WMO International Cloud Atlas, such characteristics as ceiling, layers, sky cover classification, summation amount, and others. Human observers are limited in the quantity of observations they can make, and may suffer from significant variability from observer to observer.
Automatic instruments such as laser rangers (e.g. LIDAR) have also been used to measure cloud height. Such instruments collect data in only one, or relatively few, measurement directions. In addition, no attempt is made to determine the presence and/or movement of towering cloud structures. Consequently, such instruments provide incomplete reporting of cloud cover and cloud height.